Electrically erasable programmable read only memory (EEPROM) structures are commonly used in integrated circuits for non-volatile data storage. EEPROM device structures commonly include a polysilicon floating gate formed over a tunnel dielectric, which is formed over a semiconductor substrate, to store charge. As device dimensions and power supply voltages decrease, the thickness of the tunnel dielectric cannot correspondingly decrease in order to prevent data retention failures. An EEPROM device using isolated silicon nanocrystals or nanoclusters as a replacement to the floating gate does not have the same vulnerability to isolated defects in the tunnel dielectric and thus, permits scaling of the tunnel dielectric and the operating voltage without compromising data retention.
In order to have a significant memory effect as measured by the threshold voltage shift of the EEPROM device, it is necessary to have a high density of silicon nanoclusters of approximately 1E12 nanoclusters per cm2. One method to achieve such a density of nanoclusters is to fabricate the nanoclusters using chemical vapor deposition (CVD) using disilane (Si2H6). However, the resulting nanoclusters vary in size distribution, which decreases reliability of the EEPROM devices. To improve reliability, a method to form nanoclusters with narrow size distributions at desired densities is needed.
Nanocrystal memories typically have a data storage retention time that is specified as typically no greater than ten years in time. However, there are numerous commercial products which use non-volatile memories that must have a longer retention time. Further scaling of memory devices requires a reduction in operation voltages and program and erase times.
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